Cold storage units, including walk-in coolers and freezers, have been in use for many years and have a wide variety of applications. Such units usually require a large amount of energy to operate, as the refrigeration system requires electrical energy to run a condenser, compressor, evaporator, and other components. Many refrigeration systems must operate frequently in order to maintain a desired temperature inside the storage unit, as heat may naturally enter the unit through opening doors into the storage unit, through thermal contact with a warmer external environment, and so on. Freezer units generally require even more electrical energy to operate than coolers, not only because the desired internal temperature of the unit is usually much lower than the external environment temperature, but also because additional energy is usually required to keep certain components of the freezer from undesirable freezing and icing, such as the freezer door and moisture draining system.
In most applications, cold storage units must draw their energy from an external power grid, such as a power grid system operated by a utility company. The large amounts of energy needed to operate a cold storage unit may thus translate into a large expense for any individual or business operating such a unit. Higher efficiency cold storage units may help in reducing the amount of power needed from a power grid to operate a refrigeration system, but ideally this dependence could be eliminated or reduced such that the refrigeration system could essentially draw a net energy of zero from a power grid, as for example where an independent power source operates the refrigeration system and feeds excess generated power back to the external power grid. The dependence on an external grid poses other potential problems as well. For example, during extended power outages, power may not be available from external power grids to run a refrigeration system, leading to possible spoilage of goods stored inside the unit.
Thus, there is a continuing need for the development of higher efficiency cold storage units that can operate independently of a power grid, and for cold storage units that can operate in conjunction with a power grid to draw a net zero of energy from such power grids.